This invention relates to personal care and more particularly to certain glycerol functional polysiloxanes useful as humectants in skin care applications.
The water content of the outer layers of the stratum corneum of the human epidermis is a controlling factor in the appearance of dry skin symptoms. When the stratum corneum contains an adequate amount of water within the range of about 10 to 20 percent, the skin remains flexible. However, when the water content falls below about ten percent, the stratum corneum often becomes brittle and rough and can exhibit scaling and cracking.
The stratum corneum receives its water from the deep layers of the epidermis by diffusion or when it is brought into direct contact with water. The diffusion process is controlled by the water content of the skin as well as by the concentration gradient. In a very dry environment for example, the water loss from the external skin layers can be significant and often exceeds the rate of replenishment by the diffusion process.
It is not uncommon therefore to include in skin conditioning compositions a humectant which is capable of introducing moisture to the skin from the atmosphere in conditions of moderate or high humidity. In conditions of low humidity humectants attract moisture from the lower layers of the skin. Humectants are materials which are hygroscopic and are therefore capable of retaining moisture. Among the most well known water retentive humectants capable of preventing drying out of the skin is glycerol. Glycerol is known to be an effective humectant and is generally considered harmless in cosmetic applications. It is a clear water white viscous liquid having the chemical formula HOCH.sub.2 CHOHCH.sub.2 OH. However, glycerol exhibits no bonding to the skin and hence is not durable or substantive with the result that it can be washed from the skin surface. It is used in many creams and lotions for the purpose of keeping the skin soft and for replacing skin moisture.
The prior art is replete with various formulations which contain glycerol as an ingredient. One such prior art composition is described in U.S. patent application Ser. No. 07/489117 filed Mar. 5, 1990 of Andrew H. Ward entitled "Glyceroxyfunctional Organosilicon Compounds", which application is assigned to Dow Corning Corporation. The Ward application which is now U.S. Pat. No. 5,043,359 issued Aug. 27, 1991, discloses a silicone compound having the group --OCH.sub.2 CH(OH)CH.sub.2 OH directly bonded to a silicone atom in the polymer main chain. Upon contact with water, the silicone compound undergoes hydrolysis with the result that the Si--O bond is cleaved and free glycerol is released onto the skin accompanied by the simultaneous formation of silanol groups. However, as already noted, glycerol is not durable and can be easily removed from the skin surface by washing.